Overstepping LÃ¶wenstein's RuleâA Route to Unique

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Overstepping Löwenstein’s RuleA Route to Unique Aluminophosphate Frameworks with Three-Dimensional SaltInclusion and Ion-Exchange Properties Vladislav V. Klepov,†,‡,§ Christian A. Juillerat,†,‡ Evgeny V. Alekseev,§ and Hans-Conrad zur Loye*,†,‡

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†

Department of Chemistry and Biochemistry and ‡Center for Hierarchical Wasteform Materials, University of South Carolina, Columbia, South Carolina 29208, United States § Institute for Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428 Jülich, Germany S Supporting Information *

ABSTRACT: The synthesis of four non-Lüwenstein uranyl aluminophosphates, [Cs13Cl5][(UO2)3Al2O(PO4)6], Rb7[Al2O(PO4)3][(UO2)6O4(PO4)2], Cs3[Al2O(PO4)3][(UO2)3O2], and Rb3[Al2O(PO4)3][(UO2)3O2], the first uranyl phosphate saltinclusion material [Cs4Cs4Cl][(UO2)4(PO4)5], and a related structure Cs4[UO2Al2(PO4)4], all prepared by molten flux methods, is reported. All compounds are discussed from the point of view of their structural features favoring, in some cases, ion-exchange properties. Lüwenstein’s rule, well known in the realm of zeolites, aluminosilicate, and aluminophosphate minerals, describes the tendency of tetrahedra (Al, P, Si, and Ge) linked by an oxygen bridge to be of two different elements resulting in the avoidance of Al−O−Al bonds. Zeolites and related aluminosilicate/aluminophosphate minerals are traditionally formed under relatively mild temperatures, where zeolites are synthesized using the hydrothermal synthetic technique. Few exceptions to Löwenstein’s rule are known among aluminophosphates, and four of the five exceptions are synthesized under either high temperature or high pressure methods. For that reason, the high-temperature flux synthesis of four new nonLüwenstein uranyl aluminophosphates realizes a unique synthetic approach to forming the new pyroaluminate-based building block, [Al2O(PO4)6]14−, that can be easily obtained and employed for the construction of new porous structures.

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INTRODUCTION Although alternative energy sources have been extensively studied over the past decades in order to meet ever growing energy demands of industry and attain a sustainable energy cycle, nuclear power remains one of the most important and promising power suppliers and will retain this significant role in the future, especially in light of the development of new generations of breeder reactors, such as the thorium molten salt reactor.1 The nuclear fuel cycle is a well-studied process; however, its final steps, specifically the separation and disposal of spent nuclear fuel, including control to prevent the migration of radionuclides in the environment, have not yet achieved full implementation. The solution of this problem is still to be found, and therefore actinide chemistry attracts significant interest from this applied point of view. Many known classes of inorganic materials have been proposed as matrices for the components of the spent nuclear fuel, particularly mineral-based ones. No universal storage material has been found so far, however, and the important task to develop and evaluate new candidates as host matrices remains.2 © XXXX American Chemical Society

One of the relatively new classes of materials proposed for radionuclide storage are salt-inclusion materials (SIMs), which consist of a covalent metal oxyanion framework containing voids filled by ionic salt lattices. We are particularly interested in three-dimensional (3D) porous SIMs that are prospective new hosts for the safe, long-term storage of the most abundant radioisotopes found in nuclear waste such as 137Cs, 90Sr, 129I, Pu, and U, which pose significant environmental threats due to their mobility. SIMs have the potential to sequester multiple radionuclides because of the presence of both a metal oxyanion framework that can incorporate actinide species and the salt inclusion that can contain ionic radionuclides.3 Such porous frameworks also offer additional flexibility by being good candidates for postsynthetic modification via ion exchange. This approach has already been validated by the synthesis of multiple Cs and U containing silicate and germanate SIMs including [Cs3F][(UO2) (Si4O10)], [Cs2Cs5F][[(UO2)3(T2O7)2] (T = Si, Ge), [Cs9Cs6Cl]Received: October 12, 2018

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DOI: 10.1021/acs.inorgchem.8b02906 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry [(UO2)7(Si6O17)2(Si4O12)], [Cs2Cs5F][(UO2)2(Si6O17)],4 [K3Cs4F][(UO2)3(Si2O7)2],5 [Cs6A2Cl2][(UO2)3(Ge2O7)2] (A = Ag, K), and [Cs6Cs0.71Cl0.71][(UO2)3O3(Ge2O7)].6 In order to achieve better performance of SIMs for ion exchange and improve their host properties, we aim to expand the library of uranium SIMs by introducing new framework building blocks, such as borate, molybdate, vanadate, phosphate, and aluminate oxyanions. Several phosphate SIMs are already known, although none contain actinide species, and include [BaCl][CuPO4],7 [Na2Cs2Cl2][Cu3(P2O7)2], [K2Cs3Cl3][Cu3(P2O7)2], [Cs8Cl6][Cu7(P2O7)4], [Cs5Cl3][Cu5(P2O7)],8 [CsCl][Na2Mn3(P2O7)2], [RbCl][Na2Mn3(P2O7)2], [CsCl][Na 2 Fe 3 (P 2 O 7 ) 2 ], [RbCl][Na 2 Fe 3 (P 2 O 7 ) 2 ], [CsK 2 Cl][Fe3(P2O7)2], and [CsK2Cl2][Mn3(P2O7)].9 To date, there have not been any reported aluminophosphate SIMs, although aluminophosphate frameworks are numerous.10−12 Herein, we report the first uranium containing phosphate SIM, [Cs4Cs4Cl][(UO2)4(PO4)5] (1), a layered uranyl aluminophosphate, Cs4[UO 2Al2(PO 4)4 ] (2), the first uranium aluminophosphate SIM with 3D salt-inclusion component, [Cs13Cl5][(UO2)3Al2O(PO4)6] (3), and three new uranyl aluminophosphates with 3D frameworks, Rb7[Al2O(PO4)3][(UO2)6O4(PO4)2] (4), Cs3[Al2O(PO4)3][(UO2)3O2] (5), and Rb3[Al2O(PO4)3][(UO2)3O2] (6). Aluminophosphates 3−6 reported in this paper contain [Al2O(PO4)6]14− building blocks that consist of pyroaluminate groups, where each AlO4 tetrahedron corner shares with one AlO4 and three PO4 tetrahedra (Figure 1). The presence of the

but that this could be overcome by excess thermal energy present in high-temperature methods of synthesis.10−12,15 To date, only five aluminophosphate exceptions to Löwenstein’s rule have been reported and include ultramarine,16 MAlPO5 (M = Mg, Fe),17,18 Cs2Al2P2O9,12 and Li6Na3Sr14Al11P22O90.15 The latter four compositions were synthesized under high temperature or high pressure conditions, where MAlPO5 was synthesized at 500 °C and 2000 bar and Cs2Al2P2O9 and Li6Na3Sr14Al11P22O90 were synthesized using molten flux methods with maximum temperatures of 800 and 950 °C, respectively. We synthesized structures 3−6 in high-temperature molten fluxes and they do not abide by Löwenstein’s rule, further supporting that the excess thermal energy at high temperatures can lead to Al−O−Al linkages. In addition to the scarcity of non-Lüwenstein aluminophosphates, compounds that simultaneously contain uranium, aluminum, and phosphorus are rare and all 12 reported species to date, only seven of which have known crystal structures, are minerals. The known minerals are all layered and built of autinite-type sheets (e.g., sabugalite), phosphuranylite sheets (alutiphite), or novel sheets (kamitugaite).19−24 Of the seven known crystal structures, only phuralumite, upalite, kamitugaite, and furongite contain Al−O−P linkages and all abide by Löwenstein’s rule.

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EXPERIMENTAL SECTION

Caution! Although the uranium precursors used contained depleted uranium, standard safety measures for handling radioactive substances must be followed. Synthesis. UO2(CH3CO2)2·2H2O (International Bio-Analytical Industries, powder, ACS grade), UF4 (International Bio-Analytical Industries, powder, ACS grade), AlPO4 (Alfa Aesar, powder, 99.99%), (NH4)2HPO4 (VWR, ACS grade), CsCl (Alfa Aesar, powder, 99%), and RbCl (Alfa Aesar, powder, 99.8%) were used as received. [Cs4Cs4Cl][(UO2)4(PO4)5] (1) was obtained by a reaction between 0.100 g of UO2(CH3CO2)2·2H2O and 0.0623 g of (NH4)2HPO4, molar ratio 1:2, in the presence of 2.60 g of CsCl flux in a platinum crucible. The reaction was carried out at 700 °C for 5 h and then cooled to 620 °C in 7 h. After the reaction cooled to room temperature by switching off the furnace, the flux was dissolved in distilled water and the resulting product was filtered and washed with distilled water and acetone. The product was found to consist of phase pure needle-like yellow crystals with the yield of 26% based on U. Cs4[UO2(AlP2O8)2] (2) and [Cs13Cl5][(UO2)3Al2O(PO4)6] (3) can be obtained in a reaction of 0.050 g of uranyl acetate UO2(CH3CO2)2·2H2O, 0.0156 g of diammonium hydrophosphate (NH4)2HPO4, and 0.0144 g of aluminum phosphate AlPO4 (molar ratio 1:1:1) in 0.80 g of CsCl flux in a platinum crucible. The crucible was placed into a programmable furnace, ramped up to 700 °C, held at this temperature for 7 h, and then cooled to 620 °C within another 7 h. The furnace was shut down and let to cool down to room temperature. The crystals of both phases have a block-like morphology, which makes their manual separation unfeasible. However, large crystals of [Cs13Cl5][(UO2)3Al2O(PO4)6] in the shape of a hexagonal prism can be obtained as a major phase in a reaction of UO2(CH3CO2)2·2H2O (0.050 g, 0.12 mmol) and (NH4)2HPO4 (0.0467 g, 0.35 mmol) in Al2O3 crucible in the presence of 2.00 g of CsCl as a flux. After fast ramping up to 750 °C, the reaction dwelled for 12 h and then cooled to 540 °C with a rate of 6 °C/h. Upon reaching this temperature, the furnace was shut off and cooled to room temperature. The product was washed with distilled water to dissolve flux, and the resulting mixture of both phases was filtered, washed with acetone, and dried in air. Phase pure sample of Cs4[UO2(AlP2O8)2] can be obtained by a reaction of 0.100 g of UO2(CH3CO2)2·2H2O and 0.1868 g of (NH4)2HPO4 in a 1:6 molar ratio in the presence of 0.50 g of CsCl

Figure 1. [Al2O(PO4)6]14− building block where the pyroaluminate group is blue, phosphate tetrahedra are gray, and oxygen atoms are red. Direct corner sharing between AlO4 groups breaks Löwenstein’s rule and creates a unique linear inorganic building unit.

Al−O−Al bond among the phosphate tetrahedra breaks Löwenstein’s rule, which states that aluminum silicate and aluminum phosphate structures will avoid the formation of Al−O−Al bonds in preference to each aluminate tetrahedron coordinating to four silicate (or phosphate) tetrahedra and vice versa.13 Löwenstein’s rule was developed to explain the nonrandom substitution of Al in silicate minerals and also describes trends in synthetic aluminosilicate and aluminophosphate zeolites. In the case of both minerals and zeolites, the synthesis usually takes place in hydrothermal conditions, with relatively mild temperatures of ∼100−200 °C for zeolite synthesis.14 Theoretical calculations have predicted that the formation of Al−O−Al linkages is energetically unfavorable, B


C

2 P1̅ 10.8280(4) 10.8502(4) 13.1692(5) 84.0600(10) 81.3360(10) 88.6820(10) 1521.29(10) 0.16 × 0.04 × 0.03 300(2) 4.046 2.280−27.499 15.560 30 513 6960 0.0270 −14 ≤ h ≤ 14 −14 ≤ k ≤ 14 −17 ≤ l ≤ 17 3.025 −1.676 1.055 0.0294 0.0822

0.0507

Cs4[UO2Al2(PO4)4]

1 C2/c 27.3192(10) 14.1800(5) 9.5900(4) 90 110.4000(14) 90 3482.0(2) 0.08 × 0.02 × 0.02 300(2) 5.062 2.266−28.999 27.188 100 735 4613 0.0389 −37 ≤ h ≤ 37 −19 ≤ k ≤ 19 −13 ≤ l ≤ 13 1.289 −1.782 1.051 0.0170

[Cs4Cs4Cl] [(UO2)4(PO4)5]

0.1961

3 Pnma 22.385(2) 18.172(2) 12.916(2) 90 90 90 5254.0(12) 0.10 × 0.08 × 0.06 300(2) 4.241 2.408−27.500 18.649 116 182 116 182a 0.0669 −29 ≤ h ≤ 29 −23 ≤ k ≤ 23 −16 ≤ l ≤ 16 4.472 −3.705 1.119 0.0687

[Cs13Cl5] [(UO2)3Al2O(PO4)6]

0.0841

4 P1̅ 7.0308(2) 14.2573(4) 19.7866(5) 86.3690(10) 80.3080(10) 89.6560(10) 1951.16(9) 0.04 × 0.01 × 0.01 300(2) 4.812 2.460−26.390 33.848 197 851 18961 0.0540 −11 ≤ h ≤ 11 −23 ≤ k ≤ 23 −32 ≤ l ≤ 33 4.342 −4.461 1.040 0.0418

Rb7[Al2O(PO4)3] [(UO2)6O4(PO4)2]

0.0387

5 Cmce 21.9898(10) 14.7796(6) 13.9792(6) 90 90 90 4543.2(3) 0.05 × 0.04 × 0.02 300(2) 4.666 2.209−36.342 26.450 228 284 5626 0.0446 −36 ≤ h ≤ 36 −24 ≤ k ≤ 24 −23 ≤ l ≤ 23 2.481 −1.544 1.146 0.0165

Cs3

0.0643

6 Cmce 21.9016(8) 14.4801(6) 13.9796(7) 90 90 90 4433.5(3) 0.04 × 0.02 × 0.01 300(2) 4.355 2.228−36.313 28.794 223 235 5486 0.0597 −36 ≤ h ≤ 36 −23 ≤ k ≤ 24 −23 ≤ l ≤ 23 5.754 −4.012 1.143 0.0263

Rb3

0.1135

7 Cmce 21.8703(4) 14.9507(4) 14.0742(3) 90 90 90 4601.93(18) 0.04 × 0.04 × 0.01 300(2) 4.590 2.195−25.242 25.571 228 853 5691 0.0464 −36 ≤ h ≤ 36 −24 ≤ k ≤ 24 −23 ≤ l ≤ 23 5.621 −3.334 1.268 0.0472

Cs2.5K0.5·2H2O

[Al2O(PO4)3][(UO2)3O2]3−

0.1686

8 Cmce 21.7761(6) 14.5323(4) 14.0322(4) 90 90 90 4440.6(2) 0.04 × 0.04 × 0.01 300(2) 3.999 2.224−36.350 23.306 221 394 5512 0.0559 −36 ≤ h ≤ 36 −24 ≤ k ≤ 24 −23 ≤ l ≤ 23 10.800 −7.192 1.134 0.0582

Na2.5Rb0.5·2.6 H2O

a

Reflections were not merged because of twinning; aR1 = ∑||F0| − |Fc||/∑|F0|. bwR2 = [∑w(F02 − Fc2)2/∑w(F02)2]1/2; P = (F02 + Fc2)/3; w = 1/[σ2(F02) + (0.155P)2 + 45.3606P] for 1, w = 1/[σ2(F02) + (0.195P)2 + 76.9419P] for 2, w = 1/[σ2(F02) + (0.0524P)2 + 380.5913P] for 3, w = 1/[σ2(F02) + (0.0097P)2 + 35.5190P] for 4, and w = 1/[σ2(F02) + (0.0151P)2 + 387.4263P] for 5.

number S. G. a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 crystal size (mm3) temperature (K) density (g cm−3) θ range (deg) μ (mm−1) collected reflections unique reflections Rint h k l Δρmax (e Å−3) Δρmin (e Å−3) GoF R1(F) for F02 > 2σ(F02)a Rw(F02)b

formula

Table 1. Crystallographic Information for Structures 1−8a

Inorganic Chemistry Article


Article

Inorganic Chemistry flux in an alumina crucible. The reaction was ramped up to 770 °C, held at this temperature for 20 h, and then cooled down to 590 °C in 30 h. After that, the furnace was switched off and cooled to room temperature. The product was separated from the flux by dissolving the flux in distilled water, filtered, and washed with acetone. The resulting plate- and block-shaped crystals are Cs4[UO2(AlP2O8)2], which was confirmed by powder X-ray diffraction (PXRD). As the final product contains aluminum, the only possible source of which is the reaction vessel, it appears that CsCl flux dissolves the walls of the alumina crucible, providing the reaction mixture with aluminum. The yield of the product is 72% based on uranium. In order to obtain a phase pure sample of [Cs13Cl5][(UO2)3Al2O(PO 4 ) 6 ], we performed a solid-state reaction between UO2(CH3CO2)·2H2O, CsCl, AlPO4, (NH4)2HPO4, and CsNO3 in the molar ratio 3:5:2:4:8 corresponding to the composition of the targeted compound. A sample containing 0.4240 g (1 mmol) of UO2(CH3CO2)·2H2O and the respective amounts of the other reagents were finely ground and heated to 400 °C in a quartz tube to decompose initial reagents. After the mixture was thoroughly ground a second time, it was transferred into a platinum crucible and held at 520 °C for 24 h. The purity of the sample was confirmed by PXRD (Figure S1). Rb7[Al2O(PO4)3 ][(UO2)6O4(PO4)2] (4), Cs3[Al2O(PO4)3][(UO2)3O2] (5), and Rb3[Al2O(PO4)3][(UO2)3O2] (6) were synthesized by molten flux methods25 using alkali chloride fluxes and alumina reaction vessels. For the Rb-containing materials, 0.5 mmol of UF4, 0.33 mmol of AlPO4, and 20 mmol of RbCl flux were loaded into 5 mL alumina crucibles measuring 2.6 mm high and 1.8 mm in diameter. For Cs3[Al2O(PO4)3][(UO2)3O2], stoichiometric ratios of U and P were used and 0.5 mmol of UF4, 0.5 mmol of AlPO4, and 10 mmol of CsCl flux were loaded into the same alumina crucibles. A larger inverted alumina crucible was placed over the reaction vessels in a ceramic holder in order to minimize flux volatility issues. All reactions were heated to the target temperature in 1.5 h, held for 12 h, and slow-cooled to the desired temperature at 6 °C/h. Rb3[Al2O(PO4)3][(UO2)3O2] and Cs3[Al2O(PO4)3][(UO2)3O2] were heated to 775 °C and cooled to 550 °C, while Rb7[Al2O(PO4)3][(UO2)6O4(PO4)2] was heated to 875 °C and cooled to 400 °C. After reactions cooled to the desired temperatures, the furnace was shut off and allowed to reach room temperature before sonicating the reaction vessels in water to dissolve the flux and filter out the crystalline products. All reactions produced yellow rectangular blocks in good yield (>80%) along with minor impurities and were handpicked to obtain pure samples for optical characterization and ion exchange experiments. Ion Exchange. Single crystal to single crystal ion-exchange reactions were performed on structures Cs3[Al2O(PO4)3][(UO2)3O2] (5) and Rb3[Al2O(PO4)3][(UO2)3O2] (6) by loading 20 mg of single crystals into a 1 dram vial and adding 4 mL of aqueous 4 molal KCl and 6 molal NaCl solutions to crystals of 5 and 6, respectively. The 1 dram vials were heated to 90 °C in a mineral oil bath and maintained at this temperature for 5 days. Afterward, the crystals were thoroughly rinsed and examined by single-crystal X-ray diffraction (SXRD) and the resulting ion-exchange products of 5 and 6 are Cs2.5K0.5[Al2O(PO 4 ) 3 ][(UO 2 ) 3 O 2 ]·2H 2 O (7) and Na 2.5 Rb 0.5 [Al 2 O(PO 4 ) 3 ][(UO2)3O2]·2.6H2O (8), respectively. The details on how these formulas were determined will be discussed in subsequent sections. Bulk ion-exchange reactions were also performed on Rb3[Al2O(PO4)3][(UO2)3O2] (6) by using a 30 mg sample of finely ground crystals and soaking it in 6 molal NaCl; PXRD was performed before and after the ion exchange experiment. Energy dispersive spectroscopy (EDS) was used to investigate ratios of the desired alkali metals in both single crystal and powder ion-exchange products. A control experiment was performed by soaking a 20 mg powder sample of 6 in deionized water for 5 days at 90 °C. Thermogravimetric Analysis. The water content of the bulk ion-exchange product of Rb3[Al2O(PO4)3][(UO2)3O2] (6) soaked in NaCl solution was investigated using thermogravimetric analysis (TGA). The data were collected using an SDT Q600 DTA/TGA, and the sample was heated in an alumina crucible at a rate of 10 °C/min

from room temperature to 500 °C under a 100 mL/min nitrogen flow and then allowed to cool to room temperature in air. Energy Dispersive Spectroscopy. EDS was performed on single crystals of all reported structures and ion-exchange products directly affixed to a scanning electron microscopy stub by carbon tape to verify elements present in the samples. Data were collected using a Tescan Vega-3 SEM system equipped with a Thermo EDS attachment. Optical Measurements. UV−vis and fluorescence measurements were performed on pure phase samples of 1−6 using a PerkinElmer Lambda 35 UV/vis scanning spectrometer equipped with an integrating sphere and a PerkinElmer LS55 luminescence spectrometer, respectively. UV−vis diffuse reflectance data were internally converted to absorbance using the Kubelka−Munk equation.26 Powder X-ray Diffraction. A Bruker D2 Phaser equipped with a LYNXEYE silicon strip detector or a Rigaku Ultima IV diffractometer equipped with a DTex detector, both of which use Cu Kα (λ = 1.54056 Å) sources, was used to collect PXRD data. PXRD patterns were used for product identification and to verify phase purity of samples of compounds 1−6 used for optical measurements. Single Crystal X-ray Diffraction. SXRD data were collected at 300(2) K on a Bruker D8 QUEST diffractometer equipped with an Incoatec IμS 3.0 microfocus radiation source (Mo Kα, λ = 0.71073 Å) and a PHOTON II area detector. The crystals were mounted on a microloop with immersion oil. For 4−8, the blocks were cleaved into thin plates and cut to an appropriate size. The programs SAINT+ and SADABS within the APEX 3 software were used to reduce and correct the raw data for absorption effects.27 The SHELX suite was used within the Olex2 GUI to solve and refine the structure, and specifically the SHELXT solution program was used.28−30 Crystallographic data for all compounds are listed in Table 1. The programs ADDSYM and TwinRotMat within PLATON were used to check for missed symmetry elements and minor twin components.31 Preliminary unit cell determination for the rod- and needle-like crystals of [Cs4Cs4Cl][(UO2)4(PO4)5] (1) revealed a body-centered orthorhombic unit cell with lattice parameters a = 9.60, b = 14.18, and c = 25.62 Å. Absorption correction using SADABS program in the orthorhombic crystal system results in an Rint value of 8.35%, which is slightly higher than expected 4−6% usual for uranium phosphate compounds.32 Attempts to solve the structure in the orthorhombic unit cell were not successful and did not result in a physically reasonable structural model. Diffraction data were reintegrated using the same lattice parameters with β ≈ 90° in the monoclinic crystal system. After reintegration, the Rint value decreased to 4.82%, and an initial solution was successfully found in the space group I2/a. Despite an overall improvement of the model as compared to the solution in the orthorhombic unit cell, it still contained high residual electron density peaks and severely distorted PO4 groups, along with a high R1 value of ∼16%. The model was significantly improved by a twin law (−1 0 0 0 −1 0 0 0 1), which was found by the TwinRotMat program implemented in the PLATON software and corresponds to pseudomerohedral twinning. The final structure model was refined to R1 = 1.70% in the standard C2/c setting of the monoclinic space group I2/ a, with (1 0 2 0 −1 0 0 0 −1) twin law. During initial unit cell determination for the crystals of [Cs13Cl5][(UO2)3Al2O(PO4)6] (3), a hexagonal unit cell with parameters a = b = 25.91, c = 18.21 Å was found. Full data integration in this unit cell resulted in an unreasonably high Rint value, therefore suggesting that twinning was present. An orthorhombic unit cell with parameters a = 22.39, b = 18.17, and c = 12.92 Å was found using CELL_NOW program along with two other twin components with the same unit cell parameters, both of which related to the major component by 120° rotation around the b axis.33 The data were integrated in this unit cell, and a twin law (−0.5 0 1.5 0 1 0 −0.5 0 −0.5) was found using the TwinRotMat program.31 In order to improve the quality of the final model, it was refined as a 3-component twin using the TWIN and BASF instructions. Although the resulting R1 value equal to 6.87% is rather large, the model contained physically reasonable interatomic distances and atomic thermal parameters. Rb7[Al2O(PO4)3][(UO2)6O4(PO4)2] (4) crystallizes in the centrosymmetric triclinic space group, P1̅, with unit cell parameters of a = D


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Inorganic Chemistry 7.0308(2) Å, b = 14.2573(4) Å, c = 19.7866(5) Å, α = 86.3690(1), β = 80.3080(10)°, and γ = 89.6560(10)°. The asymmetric unit is large with six uranium sites, 11 rubidium, six phosphorus, four aluminum, and 39 oxygen sites, where all atoms lie on general positions. There is disorder present in both the aluminophosphate sheet as well as the rubidium cations. All uranium and oxygen sites, as well as the P1, P2, P3, P4, and P5 sites, are all free of disorder. The P5A, P5B, Al1A, Al1B, Al2A, Al2B, Rb7A, and Rb7B sites are all 50% occupied, as the pairs (ex. P5A, P5B) are too close to be simultaneously present, and this can be explained by two different possible orientations of the aluminophosphate sheet, which will be elaborated in the Results and Discussion section. Rb4, Rb5, and Rb6, freely refine to an occupancy of 1, while the remaining rubidium sites are only partially occupied. The Rb1A, Rb1B, Rb2A, Rb2B, Rb3A, and Rb3B pairs were constrained to have a total occupancy of 1, as all the sites have occupancies less than 1 and the distances between the two sites in each pair is less than 2.6 Å. Structures 5 and 6 crystallize in the orthorhombic space group Cmce and have asymmetric units containing two U sites, one Al site, two P sites, 13 O sites, and three disordered cation sites. The 3D uranyl aluminophosphate framework is nearly identical in 5 and 6, and U2, O3, O7, O8, O9 are characterized by Wyckoff symbol 8f with m symmetry, P1 and O13 have 2-fold rotational site symmetry with either Wyckoff symbol 8d or 8e, and all other framework atoms lie on general positions. In 5, Cs1A, Cs2A, and Cs3A are assigned to Wyckoff symbols 8f, 8d, and 8e, respectively, while Rb3A is the only cation site in 6 that lies on a special position, in this case 8e. All metal atoms in the framework were individually allowed to freely refine and showed no significant deviation from full occupancies of 1. In both structures there is minor disorder in the cation sites that is easily resolved by splitting the site and/or using SUMP commands to enforce charge balance. Structures 7 and 8 are post single crystal to single crystal ionexchange samples and suffer from poor data quality due to the loss of crystal quality during the ion exchange process. The solutions for 7 and 8 are approximate but nevertheless confirm that the framework survives during the ion exchange process. In 8, the only deviation from the parent framework is the disorder of O13, which is the bridging Al−O−Al oxygen. It is clear that the electron density between the Al atoms is present and however suggests disorder that could not be accurately resolved considering the low crystal quality. The cation sites in 7 and 8 are heavily disordered, and the model of this disorder is also approximate and supports results of alkali ratios obtained by EDS and TGA data, indicating the presence of water in the channels after aqueous ion exchange. Obviously, the combination of the poor crystal quality of the ion-exchanged products and the presence of heavy elements U and Cs or Rb prevents the location of the hydrogen atoms and is of little importance to this study. In structure 7, the Cs3, Cs2, and Cs1 sites freely refine to approximately 1, 0.5, and 0.8. The half occupancy of Cs2 is chemically reasonable, considering the Cs2−Cs2 distance is 2.430(3) Å, and would be too close for two adjacent fully occupied cation sites. By letting all of these sites freely refine, there are 2.7 Cs per formula unit, which does not charge balance the framework. By modeling Cs1 as partially occupied by both K and Cs, it satisfies charge balance and freely refines to 0.401(7) K and 0.599(7) Cs. An additional smaller electron density peak remained and was too small to be a potassium site, and the modeling of it as an alkali site would not allow for charge balance, so it was modeled as a water molecule and the WA1 oxygen freely refines to 1. This model of the disorder within the channels results in 2.6 Cs, 0.4 K, and 2 H2O per formula unit. In structure 8, there are four sites total in the channel, and none of the sites in the channels could be refined as a fully occupied Rb, as none had sufficient electron density. The Rb1/Na1 site was too large to be a fully occupied Na site and is modeled as a mixture of Na and Rb with occupancies of 0.528(11) and 0.472(11), respectively. All other sites in the channel had electron densities smaller than Na. The WA1 site was of sufficient electron density to be modeled as a fully occupied oxygen atom, of a water molecule, and the Na2/WA2 site was modeled as a mixture of Na and a water molecule with

occupancies of 0.43(8) and 0.57(8), respectively. The remaining site, Na3, freely refines to an occupancy of 0.799 and was fixed to an occupancy of 0.785 in order to satisfy charge balance. This solution to the disorder in the channels results in 2.5 Na, 0.5 Rb, and 2.6 H2O per formula unit.

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RESULTS AND DISCUSSION Synthesis. Two different uranium sources were used in the syntheses, UO2(CH3CO2)2·2H2O and UF4 for 1−3 and 4−6, respectively. The main difference in the uranium sources is the oxidation state of uranium which is +6 in UO2(CH3CO2)2· 2H2O and +4 in UF4. Under the reaction conditions used, U(IV) oxidizes to U(VI). Additional reactions of 4−6 were carried out using UO2(CH3CO2)2·2H2O, as compared to UF4 to compare the impact of the uranium source on the reaction product, and the same target products were identified by powder diffraction; however, the product was a polycrystalline powder rather than single crystals produced when a UF4 source is used. No additional experiments were carried out and it is possible that further synthetic modifications could be made to result in single crystals using UO2(CH3CO2)2·2H2O as the uranium source. Although compounds 2 and 3 can be obtained in the presence of AlPO4, better yield, size, and quality of the crystals can be achieved when using CsCl flux in an alumina crucible without any additional source of aluminum. This suggests that the CsCl flux is highly reactive toward the walls of the crucible, which are thought to be inert, although there are reported instances of a flux attacking alumina crucible.34 As a result of these competing processes, the flux slowly dissolves the walls of the crucible, gradually increasing the concentration of aluminum in the system. Gradual and slow variation of one of the parameters in a chemical system, that is, concentration of the reagents, temperature, and so on, is almost always an important condition for obtaining large crystals of good quality. Indeed, the use of AlPO4 in a platinum crucible allowed us to obtain small crystals of 2 and 3, which is likely due to the readily soluble AlPO4 oversaturating the melt with respect to aluminum. Both salt-inclusion compounds 1 and 3 can be obtained under similar reaction conditions in an alumina crucible. Given their formulae, [Cs4Cs4Cl][(UO2)4(PO4)5] (1) and [Cs13Cl5][(UO2)3Al2O(PO4)6] (3), the CsCl to UO22+ molar ratios in the compositions equal 1:4 and 5:3, respectively. This allowed us to hypothesize that introducing more CsCl flux into a reaction mixture would favor 3 over 1. In addition, increasing the CsCl flux also introduces more aluminum into the system by increasing the dissolution of the reaction vessel, which also favors the formation of 3. Therefore, although it is intuitively clear that an excess of flux would help the formation of a saltinclusion phase with higher salt lattice content, it is uncertain which process is more important, because both of them favor the same product. From our experience, it is unlikely for a salt-inclusion material to be obtained by a solid-state reaction, and their preparation is generally favored by an excess of the flux. Therefore, it is quite unusual that compound 3, having a strong excess of CsCl in its composition, can readily be prepared via a solid-state reaction. This observation may serve as an indication of its stability and can be explained by the fact that all of the cesium atoms in the structure belong to the saltinclusion component. E


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Inorganic Chemistry

Figure 2. (a) sql layer topology and (b,c) related net obtained by partial removal of the 4-coordinated nodes. (d,e) Interconnection of pseudolayers in 1, resulting in a (f) complex uranyl phosphate framework accommodating cesium (blue spheres) and chlorine (green polyhedra) atoms. Uranium and phosphate polyhedra are shown in yellow and gray.

Ion Exchange. Post-ion-exchange single crystals were used to determine the crystal structures of 7 and 8 and revealed the perseverance of the uranyl aluminophosphate framework, while the sites of the species in the channels and the electron densities of those sites were different from the parent structures. As expected, the crystal quality of 7 and 8 was worse than the original samples of 5 and 6 because of the dynamic nature of the ion exchange process. For Rb3[Al2O(PO4)3][(UO2)3O2] soaked in NaCl, EDS of the single crystal used for structure determination and the bulk powder sample revealed Na/Rb ratios of 10.5:1 and 2.4:1, respectively. The TGA curve of the powder ion-exchange product shown in Figure S2 shows a 4.4% weight loss around 100 °C and a gradual 2.2% weight loss from 100 to 300 °C, where the first loss likely corresponds to surface waters and the second corresponds to the loss of water molecules from the channels. This was compared to the TGA curve of pristine Rb3[Al2O(PO4)3][(UO2)3O2] (Figure S2) prior to ion exchange showing a 3.6% weight loss around 100 °C and plateaus at 120 °C, further suggesting that the gradual weight loss between 100 and 300 °C in the post-ion-exchange curve is due to waters within the channels of the ion-exchange product. The presence of water in the pores of the ion-exchange product could be a result of the size difference between Rb and Na. The results from EDS and TGA confirm the successful exchange of Rb for Na and the inclusion of water in the pores and were used to guide the solution of the crystal structure to arrive at the approximate formula, Na2.5Rb0.5[Al2O(PO4)3][(UO2)3O2]·2.6H2O (8). A PXRD pattern of 8 demonstrates good agreement with the cif; however, there are additional unidentified peaks in the pattern shown in Figure S8. It is possible that during the ion exchange process the sample begins to partially decompose into other products that could not be identified. These diffraction peaks were not present in the control experiment, where 6 was heated in only deionized water. It is, therefore, possible that the extremely saturated salt solutions (the solutions used approach the maximum solubilities in water at room temperature) create an excessively harsh environment that causes some sample decomposition. The loss in crystallinity can also be seen in the PXRD pattern, as the Kα1/Kα2 splitting observed in the pre-ion-exchange pattern (Figure S7) is no longer present. The EDS results for the single crystal of 7 yielded an approximate ratio of Cs/K of 3:1, confirming the incorporation of K into the structures. The composition of 7 based on the

single crystal diffraction data was determined to be Cs2.5K0.5[Al2O(PO4)3][(UO2)3O2]·2H2O. The incorporation of sodium into the rubidium parent structure was more complete than the potassium into the cesium parent structure, a result that is likely due to the larger size of the cesium cation as compared to rubidium, which would hinder the removal of cesium from the pores. Similar trends in ion exchange have been recently observed in two families of layered uranyl phosphates.35,36 C r ys ta l St r u c t u r e D e s c r i pt i o n . [ C s 4 Cs 4 Cl][(UO2)4(PO4)5] (1) consists of a [(UO2)4(PO4)5]7− uranyl phosphate framework with large channels running along the c axis that are filled with salt inclusion and Cs atoms. The uranyl phosphate framework is built of UO2O5 pentagonal bipyramids and PO4 tetrahedra. The uranium coordination polyhedra contain two shorter and five longer bonds, 1.792(5)−1.799(5) and 2.270(4)−2.585(5) Å, corresponding to the uranyl and equatorial coordination bonds, respectively. Both crystallographically unique uranyl groups play the role of a tetracoordinate node, whereas one phosphate group is connected to four uranium atoms sharing each of its four vertices with uranium polyhedra (Q4 coordination type),37,38 and the other two are coordinated via one vertex- and two edge-sharing, corresponding to T12 coordination type. This coordination mode is accompanied by markedly different P−O bond distances with terminal and bridging O atoms, 1.475(6)−1.498(6) and 1.526(5)−1.566(5) Å, respectively. The salt-inclusion part of the structure consists of edgesharing ClCs6 anion-centered coordination polyhedra in the shape of an octahedron with Cs−Cl bonds varying from 3.2536(5) to 3.4845(5) Å. The edge-sharing octahedra form chains propagating along the c axis. The salt-inclusion chains occupy the central, larger part of the channels in the uranyl phosphate framework, whereas the non-salt-inclusion cesium atoms, that is, cesium atoms that are not connected directly to the chlorine atoms, are located in the windows between the channels. Some cesium atoms also reside inside the framework pores (Figure 2). In order to illustrate better the topology of the uranyl phosphate framework, it was simplified using a standard procedure39 by which the cesium cations and the salt-inclusion part were both removed, and the phosphate groups were contracted to their mass center, only retaining their connectivity to the uranium atoms through the oxygen bridges. This process reveals a 3-periodic net consisting of 3- and 4F


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Inorganic Chemistry

uranium atom U2 occupies a general position and forms two nearly symmetric uranyl bonds with lengths of 1.793(5) and 1.795(5) Å. There is an uneven length distribution among the five equatorial bonds, four of them fall into a narrow range of 2.266(5)−2.355(5) Å, whereas the fifth one, which involves a bridging oxygen atom, is elongated to 2.650(5) Å. Despite this difference, the volumes of the Voronoi polyhedra of these two uranium sites are quite similar, 9.20 and 9.24 Å3, respectively, agreeing well with the average of 9.3(4) Å3 for uranium(VI) in an oxygen environment.40 Bond valence sums of 5.99 and 6.01 are consistent with the formal oxidation state of +6 for uranium. Both aluminum and phosphorus atoms adopt a tetrahedral coordination environment. The aluminum atoms are almost uniformly surrounded by the O atoms with Al−O bond lengths ranging from 1.726(6) to 1.749(6) Å. Each phosphate tetrahedron, on the other hand, contains three longer bonds with bridging O atoms, d(P−O) = 1.529(6)−1.564(5) Å, and one short bond with a terminal O atom, d(PO) = 1.470(6)−1.489(6) Å. [Cs13Cl5][(UO2)3Al2O(PO4)6] (3) can be described as consisting of a negatively charged uranyl aluminophosphate open framework [(UO2)3Al2O(PO4)6]8− containing an extended system of intersecting channels that are occupied by the salt inclusion (Figure 4). The salt inclusion is defined here as chlorine atoms and only those cesium atoms that are directly connected to them. Given this definition, all the cesium atoms in this compound are part of the [Cs13Cl5]8+ salt-inclusion. The anionic framework and the cationic salt inclusion are electrostatically connected via ionic bonds between the cesium atoms of the salt inclusion and the oxygen atoms of the uranyl aluminophosphate framework. The uranyl aluminophosphate framework consists of uranyl, UO22+, and aluminophosphate, [Al2O(PO4)6]14− groups that function as inorganic linkers between the metal centers (Figure 4a,b). In contradiction to Löwenstein’s rule of “aluminum avoidance”,13 the aluminum atoms share a bridging oxygen atom to form an Al−O−Al fragment. Each uranyl cation coordinates four oxygen atoms from two different [Al2O(PO4)6]14− aluminophosphate groups, forming a coordination polyhedron in the shape of a distorted octahedron. The aluminophosphate groups act as dodecadentate ligands toward uranium atoms and symmetrically bind three uranium atoms at both ends of the [(PO4)3AlOAl(PO4)3]14− group. In a simplified net representation of the framework (Figure 5c), both the UO22+ metal centers play the role of two-coordinated nodes, whereas the aluminophosphate groups function as sixcoordinated nodes. Because the two-coordinated nodes do not change the connectivity of a net the uranyl groups, they, consequently, serve as a metal linker between the aluminophosphate groups, which in turn determines the topology of the net. The resulting uninodal net was assigned to the acs topology by the TOPOS software (Figure 5c).41−43 It is noteworthy that the highest possible symmetry of the net is P63/mmc. Considering that the heavier atoms of the framework, U and P, follow the hexagonal symmetry, this agrees well with the observed orthorhombic−hexagonal twinning of the crystals of this compound. The [Cs13Cl5]8+ salt-inclusion of [Cs13Cl5][(UO2)3Al2O(PO4)6] consists of face- and edge-sharing cesium chloride polyhedra. There are five unique chlorine sites, each occupying a special position with either Cs or Ci site-symmetry. While most other salt-inclusion materials contain halide sites within

coordinate nodes. This net can also be derived from a square planar sql topology in several steps. In the first step, which is shown to the left in Figure 2, some of the 4-coordinated nodes are removed to obtain larger edge-sharing 8-membered rings. It is worth pointing out that at this point there are 2coordinated nodes that do not contribute to the layer topology; however, they are important for the connectivity in the other dimension. The layer then is corrugated to bring the 2-coordinated nodes above and below the layers. Finally, the layers are connected into a 3-periodic net by sharing the 2coordinated nodes, changing the coordination of the latter to 4. The underlying net contains large 24-member rings, which correspond to the channels accommodating the salt-inclusion part, whereas the 8-membered rings are occupied by cesium atoms and play role of windows between the channels. Cs4[UO2Al2(PO4)4] (2) crystallizes in the triclinic space group P1̅ and exhibits a layered structure where the cesium cations reside between the layers. The main anionic unit of the structure is the [Al2(PO4)4]6− chain (Figure 3). In accordance

Figure 3. (a) Downlooking view of a [UO2Al2(PO4)4]4− layer in the structure of Cs4[UO2Al2(PO4)4], (b) its topology, and (c) perspective view of the structure. Uranium, phosphate and aluminate polyhedra are yellow, gray, and blue, respectively; the cesium atoms are dark blue.

with Löwenstein’s rule, each aluminum cation is connected to four different phosphate groups,13 and there are two types of the phosphate groups: those that connect three aluminum cations, therefore provide connectivity within the chains, and those that decorate the chains and attach them to the uranyl groups, thereby connecting the chains into layers parallel to the (111) plane. The negatively charged layers are connected through electrostatic interactions via the cesium atoms residing between the layers. Two crystallographically unique uranium atoms in 2 form coordination polyhedra in the shape of a tetragonal and pentagonal bipyramid for U1 and U2, respectively. U1 is located at an inversion center and forms two short uranyl bonds with d(UO) = 1.792(5) Å and four longer equatorial bonds, d(U−O) = 2.243(6) and 2.264(5) Å. The other G


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an octahedral environment, the [Cs13Cl5]8+ salt inclusion exhibits a wider set of chloride environments, including monocapped and bicapped trigonal prisms and octahedra. The salt-inclusion part of the structure fills the voids within the uranyl aluminophosphate framework and forms a 3D entity of face- and edge-sharing chlorine coordination polyhedra. Each void, as well as the channels between the voids, in the uranyl aluminophosphate framework is filled with the salt-inclusion component, which is connected throughout the framework in all three dimensions. The topology of the salt-inclusion part is therefore assumed to be dual to the framework topology. Indeed, according to the RCSR database, the dual net for acs topology is graphite gra, which can also be obtained by abstracting the salt-inclusion component from the aluminophosphate framework and its simplification by considering the chlorine atoms as nodes connected through bridging cesium atoms. The interweaving of both the framework and the saltinclusion nets is shown in Figure 5e. Rb 7 [Al 2 O(PO 4 ) 3 ][(UO 2 ) 6 O 4 (PO 4 ) 2 ] (4), Cs 3 [Al 2 O(PO4)3][(UO2)3O2] (5), and Rb3[Al2O(PO4)3][(UO2)3O2] (6) are constructed of chains of alternating uranyl hexagonal bipyramids and U2O12 pentagonal bipyramid dimers, as found in the predominant phosphuranylite topology, and [Al2O(PO4)6]14− building blocks. The uranium, aluminum, and phosphorus atoms adopt typical coordination environments and bond distances of pentagonal and hexagonal bipyramids, AlO4 tetrahedra, and PO4 tetrahedra, respectively. In the three structures, the uranyl bonds range between 1.796(3) and 1.823(5) Å, while the equatorial bonds are considerably longer at 2.221(3)−2.703(6) Å. The equatorial bonds of the 7coordinate pentagonal bipyramids have shorter bond distances between 2.261(2) and 2.422(5) Å, while the 8-coordinate hexagonal bipyramids have two shorter bonds with the oxygen shared between two pentagonal and one hexagonal bipyramid at distances of 2.221(3)−2.256(4) Å and four longer bonds between 2.482(5) and 2.703(6) Å. The Al−O and P−O bond distances average at the expected values of 1.7 and 1.5 Å. In Cs3[Al2O(PO4)3][(UO2)3O2] (5) and Rb3[Al2O(PO4)3][(UO2)3O2] (6), the aluminum and phosphate tetrahedra within the [Al2O(PO4)6]14− building blocks corner share to form a 2D network (Figure 6c), or pillars when rotated by 90°, that connect to the uranyl chains by edge-sharing through the phosphate tetrahedra. Each uranyl chain edge shares with two aluminophosphate pillars, so that each uranyl chain is separated from the next by one of these aluminophosphate pillars. This structure can also be described as uranyl phosphate sheets linked to adjacent sheets by the pyroaluminate groups, creating intersecting channels in the a and c directions. The cesium or rubidium cations lie in these channels, where one is located in the plane of the aluminophosphate sheet between phosphate tetrahedra, and the remaining cations are located between the chains of uranyl polyhedra. Rb7[Al2O(PO4)3][(UO2)6O4(PO4)2] (4), while constructed of the same building blocks, contains one fewer aluminophosphate pillar than 5 and 6. Instead of each uranyl chain being separated from the next by a pillar, two uranyl chains are connected by edge- and corner-sharing through phosphate tetrahedra, like sheets of the phosphuranylite topology, and every pair of uranyl chains is separated from the next pair by an aluminophosphate pillar. The aluminophosphate network is disordered, and there are two possible orientations of the network, as shown in Figure 6e.

Figure 4. (a,b) Main structural units, [Al2O(PO4)6] and UO2O4, in the structure of 3, and (c,d) their connection to each other resulting in (e) uranyl aluminophosphate framework.

Figure 5. (a,b) View on the framework and the salt inclusion in the structure of [Cs13Cl5][(UO2)3Al2O(PO4)6] and (c,d) their respective acs and gra simplified nets.43 (e) Arrangement of the uranyl framework and the salt-inclusion component by representing them with the dual acs and gra topologies, black and green, respectively.

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Figure 6. (a) View of 5 and 6 along the c direction. (b) View of 5 and 6 along the a direction. (c) Isolated aluminophosphate sheet in 5 and 6. (d) View of 4 along the a direction. (e) Disordered aluminophosphate sheet in 4. Uranium polyhedra are shown in yellow, phosphate tetrahedra in gray, aluminum tetrahedra in light blue, alkali cations in dark blue, and oxygen atoms are in red.

Structures Cs2.5K0.5[Al2O(PO4)3][(UO2)3O2]·2H2O (7) and Na2.5Rb0.5[Al2O(PO4)3][(UO2)3O2]·2.6 H2O (8) are ion-exchange products obtained by soaking Cs3[Al2O(PO4)3][(UO2)3O2] (5) and Rb3[Al2O(PO4)3][(UO2)3O2] (6) in KCl and NaCl, respectively. The framework is unaltered during the ion exchange process with the exception of the appearance of a minor disorder in the Al−O−Al bridging oxygen in 8, but not in 7 (Figure 8). In both ion exchange experiments, the sites of the cations and water molecules are changed (Figures 7 and

Figure 7. Comparison of the alkali metal sites in the parent structure 5 and in the ion-exchange product 7. Cesium atoms are in dark blue and the degree of transparency of the spheres approximately represents the occupancies. Oxygen atoms belonging to water molecules are teal and the shared Cs1/K1 site is dark blue and light blue.

Figure 8. On the left is the parent structure of 6 and the NaCl ionexchange product, 8, highlighting the O13 disorder in the framework and the change in alkali sites. Rubidium cations are in dark blue the degree of transparency of the spheres approximately represents the occupancies. Sodium sites are in a lighter shade of blue and oxygen sites that belong to water molecules are in teal.

8). The species in the channels tend to lie above the vertices of the uranyl polyhedra, as these sites are larger than those directly above the uranyl oxygens that align in adjacent chains and protrude into the channels. In the parent structures 5 and 6, the cations lie in approximately the same positions with additional disorder in the Cs2A/2B sites not seen in 5. In ionexchange product 8, the Rb3 site, which lies between phosphate tetrahedra in the plane of the aluminophosphate network is not present, rather there are additional sodium sites that lie above the uranyl polyhedra as compared to the parent structure. The absence of this cation site may explain the observation of the disorder in the Al−O−Al bridging oxygen in 8. Structure Building. The primary goal of this study was to investigate the possibility of introducing a salt-inclusion component into a uranyl phosphate framework and compare them to the silicate uranyl SIM materials in order to probe possible routes toward advanced SIMs with expanded ion-

exchange properties. The underlying idea behind replacing the silicate units with the phosphate building blocks to obtain new SIMs was that the charge of the PO43− phosphate group is less than that of the silicate group SiO44−, and therefore, a uranyl phosphate framework in general is more likely to have a lower charge per volume unit, allowing it to accommodate more halide atoms and increasing the fraction of the salt-inclusion component in the structure. This assumption is valid only if the condensation of the silicate units is ruled out, and only uranyl orthosilicate frameworks are considered. This, however, is rather rare because silicate groups tend to form condensed building units that result in a significantly reduced charge per silicon atom, whereas phosphate anions have significantly less tendency toward condensation. The first uranyl phosphate SIM, [Cs4Cs4Cl][(UO2)4(PO4)5] (1), reported herein indeed I


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Figure 9. Aluminophosphate building block (bottom left) when combined with different uranyl building blocks, chains, isolated polyhedra, and sheets, create structures 5 (top left), 3 (top right), and 4 (bottom right).

Scheme 1. (a) Framework Consisting of UO22+ and PO43− Groups along with Salt-Inclusion Component in [Cs4Cs4Cl][(UO2)4(PO4)5], (b) Replacement of the PO4 Units by Complex [Al2O (PO4)6] Building Units, Resulting in a Framework Accommodating 3D Salt-Inclusion Part, (c) Replacement of the UO22+ Units by Phosphuranylite Layers with (d) Further Incorporation of [Al2O(PO4)6]14− Building Units To Create (e) Rigid Framework Capable of Single Crystal to Single Crystal Ion Exchange

experiments went to completion resulting in an almost complete replacement of the salt inclusion (both Cs and Cl atoms) with the aliovalent cations from an ion exchange solution, that is, SrCl2, RbCl, KCl, NaCl, Mn(CH3CO2)2, and Eu(NO3)3. A significant drawback of this rapid ion exchange is, unfortunately, a complete loss or significant deterioration of the crystallinity in both single crystals and powder samples. Although EDS experiments show the presence of Al, U, P in the samples after ion exchange, PXRD patterns cannot be associated with the initial structures, meaning that the uranyl aluminophosphate framework either undergoes a significant change or decomposition. In order to create a framework that is more resistant to deformation during ion exchange, a more rigid uranyl-bearing can be used. Phosphuranylite layers readily form in the flux reaction conditions and therefore can be used for the uranyl unit replacement, and after some synthetic conditions optimization, this unit was introduced into the final structure. The resulting two frameworks, A3[Al2O(PO4)3][(UO2)3O2] (A = Cs, Rb) and Rb7[Al2O(PO4)3][(UO2)6O4(PO4)2], showed significantly increased resilience of the framework although the ion exchange process proceeds more slowly in these two compounds and requires elevated temperatures of 90°C. Given the very promising results of these early steps, this new building unit, [Al2O(PO4)6]14− in combination with other uranyl containing building blocks, has the potential to open up a new field of synthetic uranyl aluminophosphates. Optical Properties. UV−vis measurements of compounds 1−6 are included in the Supporting Information, Figure S9, and are typical of uranyl species with broad absorbance over

consists of isolated phosphate groups, and other numerous attempts to obtain a phosphate SIM with condensed phosphate units, that is, pyrophosphate or triphosphate, were unsuccessful. In order to improve the chances of obtaining more complex building units with a lower charge and to obtain a SIM with larger salt-inclusion fraction, an approach that has proven effective in zeolite chemistry, namely the use of aluminum together with phosphorus to create extended building units and frameworks, was employed. Using this approach, a unique aluminophosphate building block, [Al2O(PO4)6]14−, was obtained that can be combined with different uranyl building blocks, such as isolated polyhedra, chains, or sheets, to result in the complex 3D frameworks of [Cs13Cl5][(UO2)3Al2O(PO4)6], A3[Al2O(PO4)3][(UO2)3O2] (A = Cs, Rb), and Rb7[Al2O(PO4)3][(UO2)6O4(PO4)2], respectively (Figure 9). Each of these structures can also be described as consisting of uranyl phosphate sheets that are linked together by the pyroaluminate group. This new building block can conceivably be used to assemble many more new structures by combining it with other transition metal building blocks including uranium. This novel aluminophosphate building unit enabled us to build the first, to the best of our knowledge, SIM with a 3D salt-inclusion part, [Cs13Cl5][(UO2)3Al2O(PO4)6] (Scheme 1). The extended salt-inclusion component was a promising candidate for ion exchange and, in fact, multiple ion exchange experiments showed that both powder and single crystals of this compound undergo ion exchange even at room temperature. In most cases, 2-day room-temperature ion exchange J


Article

Inorganic Chemistry

specifically salt-inclusion phases, that feature unique building blocks. Characterizing the ion-exchange properties of these new uranium containing materials is important for nuclear waste storage applications. The Cs and Rb-containing structures of A3[Al2O(PO4)3][(UO2)3O2] readily ion exchange in aqueous solutions of KCl and NaCl, respectively. While the concentrations used are likely much greater than necessary for potential applications, the ability of the framework to release Cs in favor of K is unfavorable for long-term waste storage but could be useful in waste processing. The framework appears to be thermally stable up to 500 °C, suggesting that uranium aluminophosphate materials are good waste-form candidates when considering thermal stability.

200−525 nm classifying these materials as semiconductors. Fluorescence spectra (Figure 10) of 1−6 show typical yellow-

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02906. TGA data, PXRD patterns, UV−vis absorption data, fluorescence spectra, and thermal ellipsoid plot views of the samples (PDF) Accession Codes

Figure 10. Emission fluorescence spectra of 1−6 excited at 439, 439, 426, 377, 445, and 379 nm, respectively.

CCDC 1852058−1852063 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

green luminescence of uranyl-containing materials with emission peaks between 500 and 650 nm. The most intense emission peaks that occur between 525 and 550 nm can be assigned to electronic emission from the lowest vibrational level of the first excited state to the lowest vibrational level of the ground state, while the smaller peaks surrounding the main emission originate from different vibrational levels of the same electronic emission. The excitation and emission spectra for compounds 1−6 are shown in Figures S10−S15.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (803) 777-6916. Fax: (803) 777-8508.

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ORCID

Vladislav V. Klepov: 0000-0002-2039-2457 Christian A. Juillerat: 0000-0002-8270-1964 Evgeny V. Alekseev: 0000-0002-4919-5211 Hans-Conrad zur Loye: 0000-0001-7351-9098

CONCLUSIONS The synthesis of first uranyl phosphate and aluminophosphate SIMs, non-Lüwenstein aluminophosphates with ion-exchange properties are reported, namely [Cs4Cs4Cl][(UO2)4(PO4)5] (1), Cs4[UO2Al2(PO4)4] (2), [Cs13Cl5][(UO2)3Al2O(PO4)6] (3), Rb7[Al2O(PO4)3][(UO2)6O4(PO4)2] (4), Cs3[Al2O(PO4)3][(UO2)3O2] (5), and Rb3[Al2O(PO4)3][(UO2)3O2] (6). 1 and 3 are SIMs which are sought after for their potential applications in nuclear waste storage and structures 3−6 are non-Lüwenstein aluminophosphates that feature a unique building block, [Al2O(PO4)6]14−, with Al−O−Al linkages. The formation of Al−O−Al linkages in the presence of phosphorus seems to be favorable by high-temperature methods, considering the reports on the non-Lüwenstein aluminophosphates in this work and those reported by Hesse, Huang, and Yao.12,15,17,18 Löwenstein’s rule, which is a general rule based on Pauling’s rules, was developed to explain the distribution of aluminum in silicate and phosphate minerals and, in particular, was easily adapted to zeolite materials, albeit not limited to them, synthesized by hydrothermal methods. The absence of Al−O−Al linkages in zeolites and minerals and the presence of Al−O−Al linkages in materials synthesized at high temperatures suggest that Löwenstein’s rule can be overcome given enough thermal energy. The high-temperature molten flux method seems to be a viable route to synthesis new structures,

Author Contributions

C.A.J. and V.V.K. contributed equally to this manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Center for Hierarchical Waste Form Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under award no. DE-SC0016574. C.A.J. is additionally supported by an NSF IGERT Graduate Fellowship under grant number 1250052.

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